This invention relates to display devices, and in particular, to an AC-driven plasma display panel.
As known in the art, plasma display panels basically comprise a substrate with a dielectric layer thereon, and a cover, which may also include a dielectric layer, placed so as to define a gap therebetween. A gas which is capable of being ionized, such as neon with 0.1 percent argon added, is sealed within the gap. The display is defined by locally induced glow discharges in the gas produced by applying a desired potential to selected electrodes in arrays embedded in the dielectric layers.
In one form of plasma display panel, herein designated the "twin-substrate" design, a first array of parallel electrodes is embedded in the dielectric on the substrate, and a second array is embedded in the dielectric on the cover in a direction orthogonal to the first array so as to define display sites at the crosspoints of the two arrays. A desired site is displayed by applying write pulses of opposite polarities to selected electrodes in the top and bottom arrays which are sufficient to create a plasma at the crosspoint of the two electrodes. This, in turn, causes a glow discharge at the crosspoint for a short period of time. The electrons and positive ions of the plasma tend to accumulate in the site at opposite surfaces of the dielectrics so that a "wall" voltage is created and remains at the site when the write pulses are removed. The glow discharge is therefore retained at the site by applying to the two electrodes "sustain" pulses having smaller amplitudes than the write pulses and an initially reverse polarity. The sustain pulses do not have a sufficient magnitude to cause breakdown of the gas and so only sites which have previously been written will glow as a result of the wall voltage which remains from the write pulses. The sustain pulses are continuously applied as an AC signal to cause a shift in the accumulation of charge with each polarity shift and keep the site glowing until an erase signal is applied to the electrodes. The erase signal, again, includes pulses of opposite polarities applied to the two electrodes, but of a magnitude or duration which eliminates the wall voltage at the site.
The twin substrate design, although adequate, suffers from several drawbacks. The circuitry for applying the signals is fairly complex since the sustain signal is a relatively high current signal requiring application to all electrodes while the write/erase signal is a low current signal requiring application to only selected electrodes at any given time, and yet both signals are supplied by the same circuitry to the same electrodes. Further, the gap between dielectrics on the cover and substrate must be tightly controlled otherwise variations in the sustain fields at different sites will result causing glow crosstalk to unaddressed sites during sustain periods or alternatively, extinction during sustain periods of previously addressed sites. In addition, ion bombardment of the cover surface during the application of the AC sustain signal makes it impractical to include a photoluminescent phosphor on said surface to enhance the display. (For discussions of typical twin substrate designs, see, for example, U.S. Pat. No. 3,989,974 issued to Tottori et al. and U.S. Pat. No. 4,328,489 issued to Ngo.)
In order to remove some of these drawbacks, a "single substrate" design has also been proposed for AC plasma displays. In such a structure, the two arrays are both placed on the substrate and are separated by a dielectric layer. Again, display sites are formed at or near the crosspoints of the two arrays. However, since the electrodes are confined to a single substrate, the gap between substrate and cover is no longer critical, and further, a phosphor can be deposited on the cover since there is no ionic bombardment of that surface. (See, e.g., U.S. Pat. No. 4,164,678 issued to Biazzo et al.) However, the write/erase and sustain signals are still applied in essentially the same manner as the twin substrate design and so the complexity of the addressing circuitry was not reduced.
Several variations of the twin substrate design have also been proposed. For example, U.S. Pat. No. 3,989,974 issued to Tottori et al. utilizes auxiliary electrodes (25-32, 33-40) placed at both surfaces of the gas envelope and adjacent to the traditional electrodes (9-16, 17-24) previously described. The write/erase signals are supplied to the auxiliary electrodes in both substrates by means of switching electrodes (41-46, 47-52) removed from the display area, and the sustain signals are applied to the traditional electrodes. The mechanism for turn-on and erase of the display sites is not specified, but is believed to be some sort of triggering phenomenon associated with the proximity of the auxiliary electrodes to the main electrodes.
In this regard, IBM Technical Disclosure Bulletin, Vol. 23, No. 7B, December 1980, pp. 3274-3276, also describes use of auxiliary electrodes on both sides of the gas envelope which are used to sensitize adjacent crosspoint regions of the main electrodes. This can be done by any of three methods designated interstitial cell priming, capacitive coupling, and wall charge transfer mode. The first utilizes the auxiliary electrodes to produce photons at the selected crosspoint to lower the threshold of the adjacent main electrode crosspoint to cause the glow discharge. In the second method, each auxiliary electrode is capacitively coupled to an adjacent main electrode so that any pulses supplied to the auxiliary set will be coupled to the main set, while a cancellation pulse inhibits writing in non-selected regions. In the third method, the auxiliary electrodes are wider than the main electrodes so that the threshold for the auxiliary electrode crosspoints is less than the main electrode crosspoints. A combination of cancellation pulse applied to an auxiliary electrode and write pulse to the selected main electrodes selects the site to be displayed.
A further proposal for separating write/erase and sustain signals in a twin substrate design can be found in British Pat. No. 1,513,944 issued to Tsui et al. There, certain conductive lands embedded in both dielectric layers provide the sustain signal to the main electrodes by resistive coupling, while certain other conductive lands embedded in both dielectric layers provide the write/erase signal to the main electrodes by capacitive coupling.
While these proposals all provide some means for separating the write/erase and sustain signals, they all suffer from the disadvantages of the twin substrate design previously mentioned.
In the single substrate design area, proposals have been made to utilize two row conductors at each site in order to minimize external connections and simplify driver circuitry. (See, e.g., U.S. Pat. No. 4,164,678 issued to Biazzo et al.) However, to the best of applicant's knowledge, no satisfactory proposal has been made concerning how the write/erase and sustain functions can be separated in a single substrate design.
It is, therefore, a primary object of the invention to provide a plasma display structure and method of operation which maintains the benefits of a single substrate design while permitting a substantial separation of the write/erase and sustain functions.